1. Field of the Invention
The present invention involves improvements in apparatus and methods for controlling mass flow rate in rotary compressors, particularly compressors used in recuperated gas turbine engine applications.
2. Description of the Prior Art
The power output of conventional gas turbine engines can be varied by changing the turbine inlet temperatures, such as by reducing fuel flow. However, it is known that substantial increases in the part load efficiency can be achieved if the mass flow rate of the combustion air is reduced to maintain high turbine inlet temperatures, particularly in recuperated gas turbine engine applications. It is also known that precise control of the combustion temperature can lead to reductions in the amount of undesirable hydrocarbon and nitrogen oxide emissions.
Previous attempts to change or control the air mass flow rate in externally driven compressors, such as compressors driven by an electric motor or compressor section of gas turbine electrical power generator units, involve the use of guide vanes in the compressor inlet that are moveable to induce swirl to the incoming air to change the angle at which the inlet air enters the compressor blades. Most compressors are designed to have blade shapes and angles of attack orientation selected to obtain optimum mass flow rate at rated speed. These design point conditions generally presuppose inlet air incident at a fixed, predetermined angle relative to the axis of rotation. Inlet air flow incident at angles different from the design value, such as where swirl is introduced or the amount of swirl is changed, causes the mass flow rate through the apparatus to change from the design value.
A problem with conventional swirl-inducing guide vane apparatus used to vary the mass flow rate in compressors is that the maximum degree of turning or swirl achievable without substantial separation with a single set of vanes is approximately 15.degree., while variations in the swirl angle of about 30.degree. may be desirable in certain applications, such as compressors used in gas turbine engines, in order to achieve high thermal efficiency throughout the entire operating range. Although moveable vanes with turning angles greater than about 15.degree. have been attempted, these are susceptible to severe separation and consequent losses. Various attempts have been made to circumvent this problem such as by the use of two-piece articulated vanes having a fixed leading portion and a moveable trailing or tail portion. Another proposed solution utilizes two sets of vanes, a fixed set immediately upstream of a moveable set to achieve essentially the same function as the articulated vanes. These solutions are not satisfactory as less than the desired range of turning can be achieved in practice commensurate with the requirement for a reasonably low aerodynamic loss.